1. Field of the Invention
The present invention relates to devices, systems, and methods for the fixation of the spine. In particular, the present invention relates to a system that provides a low profile anterior vertebral plate and screws for fixation and stabilization of the cervical spine, the anterior cervical plate having a selectable screw locking mechanism embedded in the under side of the plate during the manufacturing thereof and providing a visual locked indicator on the top side of the plate.
2. Background of the Technology
Disease, the effects of aging, or physical trauma resulting in damage to the spine has been treated in many instances by fixation or stabilization of vertebra. The use of plates and screws for fixation and stabilization of the cervical vertebra has been widely accepted as a reliable practice and has proven to be highly successful clinically.
The various plates, which are attached to the anterior vertebral bodies of the spinal column by bone screws have some common features such as relatively flat body profiles that define multiple holes or slots through which the screws fit and are threaded into the bone. Among the differences between the conventionally used plates and screws is the manner in which the screws are locked into place in the hole or slot of the plate after the screws have been secured to the bone.
These conventional devices can be generally grouped into three basic categories with regard to how the screws are captured or secured in the plates.
The earliest plate designs can be categorized as Group I type plates. These simple designs were standard bone plates having holes through which screws were passed and screwed into the bone. These plates had no special provision for attaching the screws to the plate and as such were susceptible to having the screws back out of the plate over time. Unfortunately, there have been clinically reported instances of screws backing out of these simple Group 1 type plates with resulting surgical complications. Due to the generally unreliable nature of Group 1 type plates, fixation of the screw to the bone and to the plate is now considered as a basic requirement for vertebral plates. Due to the lack of predictable security of the screw to the plate, Group 1 type plates have fallen out of favor and have virtually disappeared from the market.
Recently efforts have been made in two generally different ways to provide features in the plates which are specifically designed to hold the screws in position once they have been inserted through the plate and screwed into the bone. One direction taken in this effort has been to design plates that incorporate retaining rings or snap features in the plates, which are configured to hold the inserted screw in place. These Group 2 type plates are very common and widely used; however, an inherent problem associated with such plates is the use of the additional very small retaining elements which can become disengaged from the plate and migrate into the surrounding soft tissues. The use of such small additional locking elements to the plates have also made it difficult to determine if the locking feature has properly engaged the screw in a locked position. Further, the difficulty experienced in accessing and disengaging the small locking elements and removing the screws from a Group 2 type plate has caused some concern for the continued use of such plates.
The other direction taken in this effort to provide plates with locking elements can be categorized as the Group 3 type plates. Group 3 type plates all provide dedicated overlying features, which are attached to plates for the purpose of covering at least a portion of the screw and thereby holding the screws in a seated and locked position. Generally Group 3 type plates are designed to provide a variety of screw covering plates, which can be selectively slid or rotated into position over at least a portion of the screw head to prevent the screw from backing out once it has been inserted. Group 3 type plates also include those mechanisms located on the top side of the plate that bear against the screw. Another less sophisticated, yet still overlying type locking mechanism, includes those plates wherein the screw is threaded through the plate in an effort to secure the screw from easily backing out. Such threaded plates simply rely on the threaded portion of the plate hole or slot to overlay the threaded portion of the screw as a locking means.
The Group 3 type plates typically require an increase in the overall composite thickness of the plate in order to accommodate the additional locking feature attached to the top side of the plate. This is a particularly unacceptable condition due to the use of such plates in an area of the spine where a thinner profile for the plate assembly is preferred. Another major problem with the Group 3 type plates is that the overlying locking element cannot always be properly positioned over the screw head if the screw was, due to anatomical necessity, positioned through the plate and into the bone at an angle such that the screw head does not fully seat in the screw recess provided on the top side of the plate. Further, if one of the overlying locking elements of a Group 3 type plate should loosen or become disengaged it is free to float loose from the top side of the plate and migrate into the surrounding soft tissue. Those simpler designs which rely only on the overlying threads of the threaded plate hole or slot to hold the screw in place are particularly difficult to use clinically in that they must be simultaneously threaded into the bone and the plate but with varying pitches and, as such, have not proven successful.
There is therefore, an unfulfilled need for a anterior cervical plate system that can maintain a relatively low profile, as found in the non-locking plates while providing the security of a locking plate system and doing so no matter how angulated the inserted screw may be. Further, there is a need for a vertebral plate that does not have the attachment of added locking elements to the top side of the plate with the predictable problems of locking elements becoming disengaged from the plate and migrating away from the top side of the plate into the surrounding soft tissue.